1. Field of the Invention
The present invention is related to a microporous polyethylene film and a method of producing the same. More concretely, the present invention is related to a microporous polyethylene film having superior extrusion-compoundability, stretchability, puncture strength, and gas permeability to increase the performance and stability of batteries using the film and a method of producing the same.
2. Description of the Prior Art
Microporous polyolefin films have been used extensively as various battery separators, separation filters, and ultrafiltration membranes owing to their superior chemical stability and physical properties.
The methods of manufacture of microporous films from polyolefins may be divided into three: The first method is processing a polyolefin into a thin fiber to produce a non-woven fabric-shaped microporous film; the second method is a dry process, in which a thick polyolefin film is made and stretched at a low temperature to create micro cracks among lamellas corresponding to the crystalline portion of the polyolefin, and eventually, to form micropores in the polyolefin; and the third method is a wet process, in which a polyolefin is compounded with a diluent at a high temperature to make a single phase, phase separation of the polyolefin and diluent is initiated in the cooling step, and the diluent is extracted to form pores in the polyolefin. Among them, the third method, i.e., a wet process, is used widely for the manufacture of separator films of the secondary batteries such as lithium ion batteries, etc. since microphorous films manufactured according to the third method are able to produce thin films, with superior physical properties.
The method of manufacture of porous films according to the wet process is further divided into the solid-liquid phase separation method and liquid-liquid phase separation method according to which steps the single phase mixtures of polymers and diluents go through for phase separation and how they make pores. Both methods are the same up to the step of making a single phase mixture by mixing polymers and a diluent at a high temperature. But in case of solid-liquid phase separation, no phase separation occurs until polymers are crystallized and become a solid. In other words, since phase separation occurs as polymer chains are crystallized and the diluent is pushed out to the outside of crystals, it is disadvantageous in that the size of the phase separation is very small considering the size of polymer crystals, and it is not possible to control the structure, such as the shape, size, etc., of the separated phase variously. In this case, the application of porous films to the secondary battery separator films having a high permeability required by high-capacity secondary batteries being developed by the manufacturers of the secondary batteries would be limited. It has been also known that there have been no ways of increasing mechanical strength other than the basic way of increasing the molecular weight of polymer resins such as mixing of ultrahigh-molecular-weight polyethylene that is costly and difficult to be mixed, and increases the load of extrusion greatly, etc. The typical composition of solid-liquid phase separation known extensively is mixing polyolefin resins with paraffin oil or mineral oil, which is introduced in U.S. Pat. No. 4,539,256, No. 4,726,989, No. 5,051,183, No. 5,830,554, No. 6,245,272, No. 6,566,012, etc.
In case of liquid-liquid phase separation, phase separation of a liquid-state polymer material and also a liquid-state diluent occurs by thermodynamic instability at a temperature higher than that of crystallization of the polymers before the polymers are crystallized and hardened to be a solid. The changes of each phases according to the conditions for phase separation and composition and concentration of each phases of phase separation have been established well in the academic field. Microporous films manufactured according to liquid-liquid phase separation are advantageous in that not only the size of pores becomes greater up to about 2 to 1,000 times than that of microporous films manufactured according to solid-liquid phase separation basically, and the temperature of liquid-liquid phase separation and the size of the phase may be controlled according to the type of the polymer and the combination of the diluent, but also the size of the phase may be controlled variously according to the difference between the temperature of thermodynamic liquid-liquid phase separation and the temperature of progressing actual phase separation, and the residence time in each step.
In U.S. Pat. No. 4,247,498, the combination of many various polymers and diluents that may be liquid-liquid phase-separated is introduced, and the possibility of making products having a thickness in an extensive range by extracting the diluent among thus liquid-liquid phase-separated compositions is described. Disclosed in U.S. Pat. No. 4,867,881 is an invention for the manufacture of oriented microporous films through stretching, extracting, drying, and heat-setting of the compositions manufactured through liquid-liquid phase separation. The methods disclosed in the above patents are limited in obtaining simultaneously superior mechanical strength and permeability that are essential physical properties for the secondary battery separators due to difficulties in offering a sufficient time for phase separation, showing effects of phase separation, and controlling pore sizes during the extrusion and cooling processes since liquid-liquid phase separation occurs in a relatively short time of a few seconds during which the resin mixture is extruded in a thermodynamically single-phase form while maintaining a temperature higher than that of liquid-liquid phase separation up to mixing and extrusion, and this molten resin material is cooled through a casting roll, etc. after it is extruded to the atmosphere. Particularly, in U.S. Pat. No. 4,867,881, nothing is mentioned specially as to the temperature of stretching in claims, but in preferred embodiments in which high-density polyethylene, the temperature of stretching is described to be lower than the melting temperature of high-density polyethylene by 20 degrees to the minimum or by 60 degrees to the maximum. In this case, a tearing phenomenon of the polymer occurs by forced low-temperature stretching, which leads to a better permeability eventually. It is deemed that a rapid increase in permeability by increasing the ratio of stretching supports this phenomenon well in preferred embodiments. However, such low-temperature stretching is deemed to be a method performed as it is not possible to obtain the structure of pores sufficiently during the processes of extrusion and cooling, and it is disadvantageous in that not only there is a high possibility of producing needle holes or abnormal-sized large holes, that are the most critical defects of battery separator products when performing low-temperature stretching, but also the danger of breakage of sheets is also increased.
Accordingly, the inventors of the present invention have conducted extensive studies in order to solve the problems with prior art, and found that it has been possible to obtain microporous films showing a high permeability by obtaining desired degree of phase separation and size of pores by controlling extensively the temperature and residence time in the phase-separated state by performing liquid-liquid phase separation in an extruder after polyethylene and the diluent are mixed into a single phase. At the same time, they completed the present invention knowing the fact that when liquid-liquid phase separation is sufficiently progressed, higher mechanical strength can be obtained even with the same molecular weight because the stretching is possibly able to be done at a temperature close to the melting temperature of polyethylene thus granting more orientation effect of polyethylene, due to the low diluent content in polyethylene rich phase.